Infinite impulse response (IIR) filters and finite impulse response (FIR) filters have been widely known as filters for use in audio processors.
An IIR filter is characterized in that it easily adjusts the gain (signal level) in any frequency band. However, an IIR filter requires a feedback loop process in computing. For this reason, computing using an IIR filter tends to accumulate computational errors. Accordingly, audio processing using an IIR filter disadvantageously shows low computational accuracy.
On the other hand, computing using an FIR filter consists of only one multiplication and one addition on each audio signal for each coefficient of the filter. For this reason, computing using an FIR filter does not accumulate computational errors and therefore shows high computational accuracy. However, in audio processing using an FIR filter, it is not easy to change only the gain in a particular frequency band. For this reason, computing using an FIR filter disadvantageously shows low frequency characteristics adjustability.
As methods to solve these problems, there have been proposed signal processors that have both the “adjustability” of an IIR filter and the “high accuracy with less computational errors” of an FIR filter (for example, see Patent Literature 1). Such a signal processor adjusts or sets a transfer function (frequency characteristics) using an IIR filter in “adjustment mode” in which any transfer function is adjusted. The signal processor then constructs a transfer function (filter coefficient) equivalent to the transfer function (filter coefficient) adjusted or set using the IIR filter, using an FIR filter in “signal processing mode” in which an adjusted or set audio signal is processed. Typically, a transfer function used in an IIR filter, FIR filter, or the like is called filter coefficient. A filter coefficient consists of a frequency range to which the filter coefficient is applied and gain values (gain coefficients) set to respective frequencies (frequency spectra).
Typically, when adjusting a transfer function with respect to an audio signal using an IIR filter, the IIR filter can be used as various types of filter. Examples of such filters include high-pass filters, which pass a high-frequency component signal, low-pass filters, which pass a low-frequency component signal, and peaking filters, which pass a particular band signal. To adjust the gain in any band using an IIR filter, a graphic equalizer including a peaking filter is typically used.